It is well known that photographic images of extremely high contrast can be formed using certain kinds of silver halides, and methods of forming such photographic images are employed in the field of photomechanical processes. For example, there is known a method of forming superhigh contrast negative images by processing a silver halide photographic material of the kind which forms latent images predominantly at the surface of the grains, to which a hydrazine derivative (one of the specified acylhydrazine compounds as described in U.S. Pat. Nos. 4,166,742, 4,168,977, 4,211,857, 4,224,401, 4,243,739, 4,272,606 and 4,311,781) is added, with a developing solution adjusted to pH 11.0 to 12.3 and containing a sulfite preservative in a concentration of 0.15 mol/l or more.
According to the foregoing method, superhigh contrast as expressed by a gamma value of above 10 can be achieved. However, the method has a defect in that fine lines become difficult to reproduce as the exposure is decreased beyond a certain limit, because the characteristic curve of the photosensitive material used shows a sharp decrease in the toe portion. More specifically, when a letter original is photographed using a superhigh contrast photosensitive material of the kind which shows a sharp decrease in a toe portion of its characteristic curve with the camera set for a condition of under-exposure, it is hard to read the developed image as letters due to a sudden drop in density of the letter image. Accordingly, such a high contrast photosensitive material suffers from the defect that its latitude in variation of exposure is narrow.
Comparatively low contrast photosensitive materials, such as those having a gamma value of below 10, do not exhibit the above-described defect. That is, there is no minimum exposure limit beyond which a sudden drop in density of letter image occurs in low contrast photosensitive materials, and the materials can gain densities high enough to be usable in the succeeding contact work even if the negative was exposed under a reduced exposure condition, and cyan provide images readable as letters. As described above, low contrast photosensitive materials have an advantage in that a latitude is extended to the low exposure side, but they have also a defect in that a high density of the background (D.sub.max) is difficult to reproduce. More specifically, since a density of the white area of an original (corresponding to the black solid area in a negative film), D.sub.max, becomes higher the higher the gamma value determined from the characteristic curve is, low contrast photosensitive materials suffer from a disadvantage of having low D.sub.max because their gamma values are low. The original characters for a block copy in the photographing of line originals may vary in density, background, and contrast, e.g., an original may be in part high in letter density and part low in letter density and contrast. For example, characters of Mingcho type (thin in line width), letters of Gothic type (thick in line width), a white, black, or colored background, and so on, may be present as a mixture in an original. Various areas of such originals would be different from one another in correct exposure setting.
When a block copy composed of those originals differing in correct exposure is intended to be photographed, the use of a photosensitive material narrow in exposure latitude makes the selection of exposure condition very difficult. Sometimes, all of the originals cannot be reproduced in a satisfactory condition by photographing at once, so the block copy is divided into some areas and partial photographing is carried out for each area under a properly adjusted exposure condition.
More specifically, when an exposure is lowered in order to reproduce black fine lines of an original, there occurs a problem that a density of the background (which corresponds to the black solid areas of the negative film, and is represented as D.sub.max) becomes low, and white fine lines (white lines in a black solid area) of the original become too thin to be reproduced. On the other hand, raising an exposure causes a problem that black lines of the original become too thin to be reproduced. Therefore, it has been desired to develop a photosensitive material having a wide exposure latitude and a high background density (D.sub.max) in photographing of line originals.